CSLB C-39 Roofing ContractorLow-Slope RoofingMedium
A general contractor is comparing two white TPO roof membranes for a cool-roof project. Membrane A has high solar reflectance but low thermal emittance, while Membrane B has both high reflectance and high emittance. Why does Membrane B achieve a higher Solar Reflectance Index (SRI) than Membrane A?
- ASRI decreases as thermal emittance increases
- BSRI is calculated using only the initial reflectance value at installation
- CSRI is based solely on membrane color regardless of measured reflectance
- DSRI combines both solar reflectance and thermal emittance, and higher values of both raise the SRI
Show answer & explanationAnswer & explanation
Correct answer: D. SRI combines both solar reflectance and thermal emittance, and higher values of both raise the SRI
SRI is a composite value calculated from both solar reflectance and thermal emittance; a membrane with high values in both properties will re-radiate absorbed heat more effectively, resulting in a higher SRI than one with high reflectance alone.
Why the other options are wrong
- A. Higher emittance actually increases SRI, not decreases it.
- B. SRI considers both aged and initial performance factors, not just initial reflectance alone.
- C. SRI is a measured performance value, not simply based on visual color.
Solar Reflectance Index (SRI)
SRI is a single value combining solar reflectance and thermal emittance to indicate how well a roof surface stays cool under sunlight.
- Higher reflectance and higher emittance both raise SRI
- Used in Title 24 and cool roof code compliance
- Black roof SRI = 0, white roof SRI = 100 (standard reference)
Memory trick: Reflect it and release it to score high SRI.